1. Field of the Invention
This invention relates to the chemical treatment of cellulose fibers to impart the fiber with higher hydrophobicity and/or durability. More particularly, this invention relates to cellulose fiber reinforced cement composite materials using sized cellulose fibers, including fiber treatment methods, formulations, methods of manufacture and final products with improved material properties relating to the same.
2. Description of the Related Art
Ordinary Portland cement is the basis for many products used in building and construction, primarily concrete and steel reinforced concrete. Cement has the enormous advantage that it is a hydraulically settable binder, and after setting it is little affected by water, compared to gypsum, wood, wood particle boards, fiberboard, and other common materials used in building products. This is not to say that water has no effect on cement. Some dissolution of chemical components does occur when cement is saturated with fresh water, and these can be transported and re-deposited in different places if the cement is once again dried.
Asbestos Fiber Cement Technology
About 120 years ago, Ludwig Hatschek made the first asbestos reinforced cement products, using a paper-making sieve cylinder machine on which a very dilute slurry of asbestos fibers (up to about 10% by weight of solids) and ordinary Portland cement (about 90% or more) was dewatered, in films of about 0.3 mm, which were then wound up to a desired thickness (typically 6 mm) on a roll, and the resultant cylindrical sheet was cut and flattened to form a flat laminated sheet, which was cut into rectangular pieces of the desired size. These products were then air-cured in the normal cement curing method for about 28 days. The original use was as an artificial roofing slate.
For over 100 years, this form of fiber cement found extensive use for roofing products (slates, and later corrugated sheets), pipe products, and walling products, both external siding (planks and panels), and wet-area lining boards. Asbestos cement was also used in many applications requiring high fire resistance due to the great thermal stability of asbestos. The great advantage of all these products was that they were relative lightweight and that water affected them relatively little, since the high-density asbestos/cement composite is of low porosity and permeability. The disadvantage of these products was that they were brittle and the high-density matrix did not allow nailing, and methods of fixing involved pre-drilled holes.
Although the original Hatschek process (a modified sieve cylinder paper making machine) dominated the bulk of asbestos cement products made, other processes were also used to make specialty products, such as thick sheets (say greater than 10 mm). These processes used the same mixture of asbestos fibers and cement as the Hatschek process. Sometimes process aids are needed in other fabrication processes, for example, extrusion, injection molding, and filter press or flow-on machines.
Two developments occurred around the middle of the last century that have high significance to modern replacements of asbestos based cement composites. The first was that some manufacturers realized that the curing cycle could be considerably reduced, and cost could be lowered, by autoclaving the products. This allowed the replacement of a portion of the cement with fine ground silica, which reacted at autoclave temperatures with the excess lime in the cement to produce calcium silica hydrates similar to the normal cement matrix. Since silica, even when ground, is much cheaper than cement, and since the autoclave curing time is much less than the air cured curing time, this became a common, but by no means universal manufacturing method. A typical formulation would be about 5-10% asbestos fibers, about 30-50% cement, and about 40-60% silica.
The second development was to replace some of the asbestos reinforcing fibers by cellulose fibers from wood or other raw materials. This was not widely adopted except for siding products and wet-area lining sheets. The great advantage of this development was that cellulose fibers are hollow and soft, and the resultant products could be nailed rather than by fixing through pre-drilled holes. The siding and lining products are used on vertical walls, which is a far less demanding environment than roofing. However, cellulose reinforced cement products are more susceptible to water induced damages, compared to asbestos cement products. A typical formulation would be about 3-4% cellulose, about 4-6% asbestos, and either about 90% cement for air cured products, or about 30-50% cement, and about 40-60% silica for autoclaved products.
Asbestos fibers had several advantages. The sieve cylinder machines require fibers that form a network to catch the solid cement (or silica) particles, which are much too small to catch on the sieve itself. Asbestos, although it is an inorganic fiber, can be “refined” into many small tendrils running off a main fiber. Asbestos fibers are strong and stiff, and bond very strongly with the cement matrix. They are stable at high temperatures. They are stable against alkali attack under autoclave conditions. Hence, asbestos reinforced fiber cement products are themselves strong, stiff (also brittle), and could be used in many hostile environments, except highly acidic environments where the cement itself is under chemical attack. The wet/dry cycling that asbestos roofing products were subjected to, often caused a few problems, primarily efflorescence (efflorescence is caused by the dissolution of chemicals inside the products when wet, followed by the deposition of these chemicals on the surfaces of the products when dried). Efflorescence caused aesthetic degradation of roofing products in particular, and many attempts were made to reduce it. Because the matrix of asbestos reinforced roofing products was generally very dense (specific gravity about 1.7), the total amount of water entering the product even when saturated was relatively low, and the products generally had reasonable freeze thaw resistance. If the density was lowered, the products became more workable (for example they could be nailed) but the rate of saturation and the total water absorption increased and the freeze thaw performance decreased.
Alternative Fiber Cement Technologies
In the early 1980's, the health hazards associated with mining, or being exposed to and inhaling, asbestos fibers started to become a major health concern. Manufacturers of asbestos cement products in the USA, some of Western Europe, and Australia/New Zealand in particular, sought to find a substitute for asbestos fibers for the reinforcement of building and construction products, made on their installed manufacturing base, primarily Hatschek machines. Over a period of twenty years, two viable alternative technologies have emerged, although neither of these has been successful in the full range of asbestos applications.
In Western Europe, the most successful replacement for asbestos has been a combination of PVA fibers (about 2%) and cellulose fibers (about 5%) with primarily about 80% cement. Sometimes 10-30% of inert fillers such as silica or limestone are in the formulation. This product is air-cured, since PVA fibers are, in general, not autoclave stable. It is generally made on a Hatschek machine, followed by a pressing step using a hydraulic press. This compresses the cellulose fibers, and reduces the porosity of the matrix. Since PVA fibers can't be refined while cellulose can be, in this Western European technology the cellulose fiber functions as a process aid to form the network on the sieve that catches the solid particles in the dewatering step. This product is used primarily for roofing (slates and corrugates). It is usually (but not always) covered with thick organic coatings. The great disadvantage of these products is a very large increase in material and manufacturing process costs. While cellulose is currently a little more than asbestos fibers of $500 a ton, PVA is about $4000 a ton. Thick organic coatings are also expensive, and hydraulic presses are a high cost manufacture step.
In Australia/New Zealand and the USA, the most successful replacement for asbestos has been unbleached cellulose fibers, with about 35% cement, and about 55% fine ground silica, such as described in Australian Patent No. 515151 and U.S. Pat. No. 6,030,447, the entirety of which is hereby incorporated by reference. This product is autoclave cured, as cellulose is fairly stable in autoclaving. It is generally made on a Hatschek machine, and it is not usually pressed. The products are generally for siding (panels and planks), and vertical or horizontal tile backer wet area linings, and as eaves and soffits in-fill panels. The great advantage of these products is that they are very workable, even compared to the asbestos based products, and they are low cost.
However, cellulose fiber cement materials can have performance drawbacks such as lower resistance to water induced damages, higher water permeability, higher water migration ability (also known as wicking) and lower freeze thaw resistance when compared to asbestos cement composite material. These drawbacks are largely due to the presence of water conducting channels and voids in the cellulose fiber lumens and cell walls. The pore spaces in the cellulose fibers can become filled with water when the material is submerged or exposed to rain/condensation for an extended period of time. The porosity of cellulose fibers facilitates water transportation throughout the composite materials and can affect the long-term durability and performance of the material in certain environments. As such, conventional cellulose fibers can cause the material to have a higher saturated mass, poor wet to dry dimensional stability, lower saturated strength, and decreased resistance to water damage.
The high water permeability of the cellulose reinforced cement materials also results in potentially far greater transport of some soluble components within the product. These components can then re-deposit on drying, either externally, causing efflorescence, or internally, in capillary pores of the matrix or fiber. Because the materials are easier to saturate with water, the products also are far more susceptible to freeze/thaw damage. However, for vertical products, or eaves and soffit linings, and for internal linings, none of these water-induced disadvantages are very relevant.
To summarize, the replacement of asbestos in Europe has been largely by air cured fiber cement products, using PVA fibers, and pressed after forming in the green state. The primary problem with this technology is increased material and manufacturing cost. The replacement of asbestos in USA and Australia/New Zealand has been largely by autoclaved fiber cement products, using cellulose fibers, and formed with lower density without pressing. The primary problem with this technology is increased rate, and quantity, of water absorption into the product when wet, and reduced resistance to freeze thaw cycles.
Certain prior art references teach using fibers that are grafted with a silane or silylating coupling agent. However, these references are directed to improving the bonding between the fibers and the cement so as to increase the strength of the composite material. As such, the coupling agents selected contain primarily hydrophilic functional groups with the specific purpose of bonding with both the hydroxyl groups on the fiber surface and the cementitious matrix. In fact, these references teach away from using coupling agents having hydrophobic functional groups as the hydrophobic groups would slightly decrease, rather than increase, the material strength.
For example, U.S. Pat. No. 5,021,093 teaches grafting a silyating agent to the fiber surface so as to improve the strength of the resulting composite material. The silyating agent comprises molecules containing hydrophilic groups on both ends so that one end can bond with hydroxyl groups on the fiber surface and the other end can bond with the cementitious matrix. The silyating agent essentially serves as a coupling agent that connects hydroxyl groups on the fiber surface to the cementitious matrix.
U.S. Pat. No. 4,647,505 teaches applying a chelating agent to a cellulose fiber to reduce fiber swelling in aqueous and alkaline solutions. The fibers are impregnated with a solution of a titanium and/or zirconium chelate compound. The chelate compound, however, does not react upon contact with the fiber, because the fiber is contained in an aqueous medium, and the chelate compounds described in the patent resist hydrolysis at ambient temperatures. Therefore, this patent describes heating the fibers above 100° C. to dry the fibers, thereby allowing the reaction to take place. After drying, the chelate compound(s) react with hydroxyl groups on the cellulose fibers to produce cross-linking between the hydroxyl group residues.
As U.S. Pat. No. 4,647,505 is directed primarily to reducing swelling of cellulose fibers, it is not specifically directed to increasing hydrophobicity of the fibers. Moreover, this patent provides an approach to fiber treatment which requires drying of the fibers in order to induce reaction with the cellulose fibers.
Accordingly, what is needed is an efficient method for preventing damage and degradation to a fiber cement building material, particularly due to water and/or other environmental effects. What is also needed are material formulations and products having improved resistance to water and/or environmental degradation.